Protective relay system



Aug. 17, 1943.

R. M. SMITH PROTECTIVE RELAY SYSTEM Filed June 2l, 1940 WITNESSES: INVENT'OR 7? @y M mf/Lb.

ATT EY Patented Aug. 17, 1943 Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 21, 1940, Serial No. 341,609

k'7 Claims.

This invention relates toV electrical protective systems and it has particular relationto systems for protecting a polyphase electrical circuit againstfground faults occurring therein.

.'In protecting electrical circuits, it is common practice` to employ directional ground current relays. When .properly;insta1led, such `relays adequately protect the system against ground faults occurring therein and serve to isolate the portion adjacent the ground from the remainder oi'y the system. Occasionally, however, such ground current relays have been found to operate .incorrectly and no satisfactory explanation of the incorrect operation could be given.

In treating polyphase alternating .current systems, it is convenient to analyze a system in terms .ofl sequence quantities, as explained in the book on symmetrical Components" vby C. M.' Wagner andrR. D; Evans, published in 1933 by McGraw-Hill Book Company, New Yorkqcity. According to Symmetrical Componentsy an unbalancedpolyphase ksystem of currents may be represented by' three symmetrical systems of r currents.` These symmetrical systems include a positive phase sequence or positive sequence component, a negative phase sequence or nega tivefsequence component, and` a zero phase sequence or zerosequence component. The directional groundrcurrent relays above referred to are'responsive to zerosequence currents.

Because of space limitations and economy, it is often desirable to locate circuits adjacent to ach ,'otlfier.` For example, two independent circuits may be mounted upon common transmission towers. {.Und'er such conditions I have found that, if a rSt polyphase circuit offers a path for. zero sequence current, substantial zero sequence current will be .induced therein when a zero sequence current flows in the second circuit. If conventional directional ground current relays are employed in the first circuit; they may De actuated by the inducedzero sequence ,current despite the fact that no ground fault is present on the rst circuit; This explains many ofthe incorrect operations ofdirectional vground currentrelays above referred to. Y Y

In Vaccordance with this invention, conventional directional ground current relays for circuits subject to zero sequenceinduced currents are replaced by directional negative sequence relays. When -a fault occurs from a phase conductor to ground on ya grounded neutral polyphase system, not -only does substantial zero se-V quence current ow, butthe resulting imbalance of the system produces a substantial amount (ci. lis- 294) of negative sequence current. In accordance with standard practice, the negative sequence current may be treated as originating at' the fault as a source and flowing in the system away from the. fault. The presence of this current permits 4the utilization of negative sequence relays.

By employing negative sequence current for energizing. the directional relays, substantially all trouble from induced current is eliminated. For example, in a three-phase system, the negative sequence currents may be represented by three equal and symmetrical vectors displaced fromv each other by Because of the balance of these vectors, substantially no negative sequence current is induced in a circuit adjacent the three-phase circuit, This is particularly,

true because of the practice of transposing the phase conductors of distribution circuits at frequentintervals in order to minimize unbalances between adjacent circuits resulting from mutual inductance. Since substantially no negative sequence current is transferred to a circuit by mutual induction, it follows that incorrect relay operation cannot be obtained from induced negative rsequence current. Moreover, since these relays are not responsive to zero sequence current, the presence of induced zero sequence current has no effect thereon.

, The directional negative sequence relay may be employed for controlling the energization of a time delay relay which is suitably energized as by zero sequence current,vor the zero sequence relay may be'employed for rendering the negative sequence relay effective.

For some circuitsit is desirable to discriminate between negative sequence current produced by internal and external faults. For example,` ina polyphase loop circuit, if apparatus capable of supplying negative sequence current is energized from the loop circuit, a fault occurring external to the loop circuit may result in the ilow of suilicient negative sequence current to operate the negative sequence relays. However, under these circumstances the magnitude of the negative sequence current would be substantially less than that flowing in the loop circuitin response to a fault occurring in the loop circuit. For this reason a negative sequence overcurrent relay may be employed for preventing energization of the'zero sequence relay or for otherwise inhibiting operation of the mechanism controlled by the directional relay unless the negative sequence current exceeds the values present kfor external faults. The provision of lample.tliefleg 6 and thefeedercircuit 3 able to the provision of a similar control in the directional relay itself. Y

It is accordingly an object of this invention to provide a circuit subject to zero sequence induced current with ground fault protective means which is-not subject to the induced current. Y

point X. As a'result of this fault asubstantial VL't is a further 'object of the invention to Vprovide a 'circuit subject to zero phase induced current with negative sequence directional relays for protecting the circuit against ground faults.

it is a still further object of this inventionto i provide la loop circuit subject to zerosequence induced current witha negative .sequence idirectional relay responsive to ground faults occurring f only withinthe loop circuit.

Still another object of this invention is to provide a circuit subject to zero sequence induced spcnsive to-the `directicn oi" negative sequence'l current, the magnitude 'of negative sequence'cure 3. Zero sequence currents iiowingv in the'thr'ee.

phase conductors of a three-phase circuit are ink phase with each other. Consequently the eiectsl Y Y of such zero sequence currents in mutual induc-i tion vare cumulative.` sequence currents owing inthe feeder circuit 3 have substantial flux linkages with the leg 6 off the loop circuit 5.

It will benoted that the loop circuit 5 consistsj of al completely closed electrical circuit. As a1 @result of the flux linkages between the feeder cir icuitt` andthe legf, a substantial zero sequence current flows inthe closed loop circuit.

Regardless of its direction of flow a zero se- V quence V'current inducedV in the loop circuit 5 current with ground fault protective means rerent andfthe magnitude of zero sequence current; 1

in saidcircuit; Y

Otherv objects ofthe invention will be appar+ entirom 'the following' description taken in 0011- junction withA the 'accompanying'.drawing, irr

Which: y

Figure l isV afdiagrammatic single-line View showing "a polyphase distribution system',V and *Fig 2 is a diagrammatic view showing a relay n system embodying"this'invention.

` Referring to the drawing, Figl'shows in single line a three-phase distribution systemve'nergized from a grounded neutralfsource such as a star connectedy three-phase generato# L'. The generator is connected to abus 2 which supplies Yenergyto a radial feeder vcircuitV 3" througl:1 a

suitable protector l and toa loop feeder circuit 5 having aj legtf afle'g 'l andatie-portion 8.'V

` .Ai-,suitable intervals around they loop circuit 5V transformers s, iii, il and I2 areprovidedtor transmitting energy from 'the loop circuit to secondaryfdistribution circuits, or to consumers.V 'v

When a ground fault occurs Aon a portion'of the loop circuit ,5, itis desirable that' only the f aulted portion or the loop circuit'be removedY from serviceso thatenergy'rnay becontinuouslx7 supp-lied over'the sound portions 'of the loop'circuit. For this purpose, suitable protectors I3 to f 22 vare,provided for the loop circuit. 'Each ofthe t protectors may includel a circuit breaker 4and 'a 'directional time" delay relay for cont-rollingtheV VtrippingY of the circuitb-reaker.Y Adjacent each Vof 'y the protectors in Fig. 1; an'arrow' indicates the direction of current flow for vwhich theprotector trips and a number adjacent the arrow indicates the time delayin 'seconds required before the protector trips. Itjis to be understood thatjthe specic protector arrangement 'anditime delays arejernployed merely vin an illustrative .Ina-nner and may vary widely'in accordance 'with the re quirements V for eachdistributionV systerr'lpfv It will bemoted in Figgl that the leg 6 of the loop circuit is adjacent theY radial feeder circuit Eifer asubstantial portion of its lengtl'i.. Forexrnay be mounted ncomrnon transmission towers. ,lliitli f circuits so close toA each other substantial mutual induction exist between the circuits( Belone or more secondary distribution circuits.

V tendstoactuate at least half ofthe protectors yassociated 'with the loop circuit.-V Depending on the .duration of the flow of such current one 'or 'more' Vofv theprotectors `may trip. to disconnect portions ofthe-loop circuit.v Y. j f

Tripping off' loop vcircuitv protectors under., such conditions is highly objectionable. VSincethe loop tern and may disrupt the continuity ofv service to If .the protectors associatedfwith the loopv circuit 5 are not responsive to zero sequence current, this objectionable tripping is prevented. y

In accordance 4with this invention each of the protectors'associated with the loop circuit is pr`o` vided with directional `tripping means responsive 'to negative sequencequantities.l A suitable protype is illustrated diagrammatic'ally tector'of this in Fig-.2.1 i

In Fig. '2 aprotector 'is illustrated'whichv includes a circuitbreakerZS having a tripping solenoid '2 4 and a' trip coil251as'sociatedv therewith.

Y Energization of the'trip Vcoil v2li-is controlled by a relay assembly which includesa relay 26 of the conventional inductiondisk type." The relay 26 comprises a potentialpole 21 having' a potential Winding` -28 associated therewithfInfadditionythe windings 3@ associated therewith.Y 4vWhenthe cilrrent and potential windings are properly; energized, a shiftingmagnetic iield is produced inthe tacts 3l associated therewith and normally biased cause'of thi'sfrnutual induction a current flowing in one of theA circuits 'under' certainv conditions may result in the flow of an induced currentin the other circiflit. @Let it be assumed that in an open conditionby means of a suitable spring 3?.; The construction of the portion of the relay 25 thus far described is well known in the art. f

In accordance with thisinvention the potentialwinding 28 is energized in accordance with the f negative sequence voltage `rin the loop circuit 5 andthel current Ewindingsill' are energized inaccordance with the negative sequence current Vflowingrinthe loop` circuit 5. To this 'end thefpotential winding 28 .is connected throughf suitable conductors 33 and 34 to the output terminals Vof a :negative sequence potential filter 35. The `current Vvviiodirigs Silare connected through suitable c onductcrs and 31 to the output terminals of a suitable negative sequence current filter 38;

a ground ramt occurs' on 7s Ihe potential lter 3,5 conveniently may be of the type illustrated in the Lenehan` Patent' 1,936,797 which is assigned to the` Westinghouse aphase conductor of thefeeder circuit 3 at a;l

For this 'reason the zeromism Electric & Manufacturing1I)y. Briey, this potential lter includes anto-transformer it! having a 40% tap 40, aninductanceal'l and 1a. .re-

sistance 42.` The inductance 47| vand the 4resistance 42;are so proportioned that the voltage drop across the resistance is .approximately 50% of the resultant lvoltage drop'across theinductance and the resistance in series and lags :the resultant voltage drop by "60, With the yparts lconnected as shown in Fig. 2 and the phase rotation A, B,C, the output of the potential Vfilter 35 is .proportional to the `negative sequence Apotential of the loop circuit 5. Further details of @the :potential filter v35m-ay be found by reference to the Vaforesaid Lenehan patent. y

The '.currentdflter 38 may :be of the type disclosed i-n the `T lenfrahan `Patent 2,161,829 which is assigned to the Westinghouse Electric -& Manuf acturing Company. This lter includes a Inutual reactor comprising two Yprimary windings 43 and 44 and a common secondary winding v45. Each of these windings has one terminal connectedto a separate .terminal of star connected current `transformers 46, 4l and 48. The -free ends of the primary windings 43 and 44 lare connected together to one terminalrofa `resistance 49, A second resistance 50 has one terminal connected to that terminal of the current transformer d8 whichalso is connected `to the secondary winding 45. As illustrated in Fig. .2, the free terminals of theresistors 'grand 5i! areconnected in parallel through conductors .5I and 52 tothe neutral .of the star connected current transformers. The polarity markings of .the reactor windings are indicated by crosses. If `the resistance 49 has va value v d the resistance 50 has avalue and the neutral vimpedance between each of A'the primary windings andthe secondary winding 65 is equal to d? the filter 3B connected, :asxshown will .deliver .an output to the conductors 35 .and 3.1 vwhich .is proportional to .negative sequence Acurrent vflowing 4in the loop circuit 5. Further .detalls :of the circuit filter 38 will lbe foundv in the :last mentioned Lenehan patent. l

Since the relay 26 is essentially a watt `responsive relay,` the direction of rotation lof its armature disk is responsive to the direction of fiow of negative sequence current in the loop circuit. Consequently the contacts 3lV will close only when the direction of flow ofnegative sequence current is in the direction for which the relay L26 is :adjusted.

vIt is desirable that the'relay .2S be a sensitive relay. For example, kunder some, conditions `the relay 26 may beenergized .byra substantial negative sequence current when only/a small negative sequence potential is available .energizing the'potential winding 28. In order to respond satisfactorily under ysuch conditions, the Arelay `26 should have a sensitive setting. l On some'di'stributio'n systems having `a 'loop circuit'the sensitive setting oi therelay 25 may .result in closure of `the vcontacts V3| 'when afault occurs'on the system :external to the loop circuit with which the Vrelay is associated. Referring to Fig. 1 lot it be mumed'that apparatus 53-capable of supporting the ilow of .a negative se qucncecnrrent is associated with one of the secondary distribution circuits such as that associatedwith the :transformer 4I l. 'For example,A the apparatus 53 may Abe a synchronous motor. When aground fault :occurs at the point X, negative sequence current will flow through the loop circuit '5 .becauseof the rpresence of the apparatus 53. Such current may .be'suficient to actuate the sensitive relays '-26.

It Vmay be possible to prevent voperation of the relays 26 under these circumstances by reducing theirvsensitvities butthissolution is objectionable iorreasons :previously 4set forth. In accordance with another solution, a separate negative se* quence overcurrent relay 54 may be associated with thedirectional relay 26. As illustrated, the overcurrent vrelay V54 islof vthe simple solenoid type. Other'forms oi overcurrent relays may lne substituted therefor. Energization for the relay 54 may be yprovided by` connecting the relay 54 in series with the windings :30 for energzation yfrom the current filter 38. When the negative sequence current `exceeds a'predeterrnined value.' the rela i4 picks up to close its iront contacts. The current value :at which the relay 54 closes its: contacts preferably is adjustable. Any suitable `adjustment for the relayf'54' may be provided. Y.lts illustrated, :an adjusting screw 55 is positioned below kthe 'relay to control the deener gized position of the relay solenoid. v

It will fbenoted Vthat the contacts of thefrelay 54 are in series vwith the `contacts of the relay 26. Consequentlysacontrol action is possible only when the contacts of both relays are closed.

Although the contacts of'the relay 2s alone, .or the contacts of therelays :54 and 26 connected in series Amay tbe employed .for certain control operations, it is Ipossible that these relays may close when .no ground fault occurs on the loop circuit .5. l'o prevent tripping Vof the circuit breaker 23 .under these conditions `an additional relay :56 is-provided for energization in accordance with zero .sequence `current iowing in the loop circuit 5.V

Although the relay 56 may take various forms, it is illustrated -as an overcurrent relay of the familiar Vinduction disk type. This relay includes a main pole 5l 'providedwith an energizing winding A58 and auxiliary poles58 provided with'auxiliary windings iii).V The auxiliary windings are connected for energization in accordance with the energization of Vthe energizing winding 5B through a torque compensator `transformer 5I.

The-auxiliary windings VGil are connectedto a secondary windingBZ on the transformer EL The transformer 5I hasa primary winding 3 which is connected for energization from a winding 64 which is also 4mounted on the pole 5'! in inductive yrelationship relative to the energizing winding 58. 1

It will be noted that the energizing circuit for theprimary winding 63 is completed through the contacts of the relays 26 and 54. Consequently the auxiliary windings '60 are effective to produce a shifting magnetic field for the relay 56 only when the contacts of both of the relays 26 and 54 are closed. VCurrent; proportional-to the zero sequence current fiowing in the loop circuit 5 may be applied to the energizing winding 58 in any manner as by including the winding in the neutral connectionof the .star connected current transformers e 46, 41 and 48. In Fig. I2 this is illustrated by the attachment of thev conductors 5i' and-zto theenergizingwinding. f Y. When the relay Sais properly energized; an armature associated therewith is actuated to close contacts .65 carriedY by the relay against -the resistance of a biasing spring 66. The relay-55 f may be provided with a time delay forv furnishing theA timevdelay desired for each protectorof the loop circuit 5., Closure of thecontacts '65' may bejemployed for anycontrolroperation. For -eX- ample,closure of the contacts may connect the winding 25 to aisource ofjdirect current for trip ping the circuit breaker V23. v 1 s i y y Three conditions must be fullled before-the contacts [i5V can close. First, the? direction of now of negativesequence "current in the loop VYcircuit 5 must be in the proper direction for actuating the relay Y26 toclose itsv contacts; second, the'V magnitude of the negativesequence current nowing in the loopr circuit 5`must `be sunicient to inthe loop circuit 5 Yfor.y the time necessary -for actuating. the relay 56 to'close its contacts.'

1t is believed that the'operation of the'sys'tem thus far described is apparent'.` 'Ifa ground fault occursonv the feeder circuit 3,` such as at the point X, substantial zero sequence current is in` Y ducedin the loop circuit 5. However, since the relay 2S is not responsive torzcero sequence current, such current does not actuate the protectors of the loop circuit.V i

Because of the selectivefaction afforded by the varyingtime delays onlythose circuit breakers adjacent the "fault trip; Although the invention has beenv described with reference to certain specic embodiments thereof, the inventionis capable of numerous modications.V Therefore, the invention is not to be restricted exceptas requiredrby the Vappendedvclaimswhenrinterpreted in view of the Y I'fclaimv as my invention: Y

l. An. alternating current system comprising apolyphase loop-circuit, a circuit adjacent said Y' polyphase loop circuit and leiiectiveunder ceractuate the' relay5'4 to close its-contacts; -and Y third sufficient zero sequence fcurrent must now If 'a Vsource capable-'of supportingnegative `se-A A quence currentis associated with'the loop circuit some 'negative sequenceY current maylow in the loop circuitV but themagnitudeV ofsuch cur= rent will be relativelyv small. This-is for the vreasons that'negativ'e sequencercurrents owing through external faults must divide between'the parallel paths offeredgby the loop circuit.Y More'- over.- the impedance offered to such-negative sequence currentg'enerally'is greater than that offered to'negative sequence current nowing to internal faults. In additiomthe-capacity of the source I generally is substantially greater than that of any apparatus associated with the sec'v ondarydistribution circuitsY and tends to support. greater negative sequencecurrent for an'internal fault lthan does the apparatus fora'fault eX- ternal to the" loop circuit.vv Ihese magnitudes of negative sequence current are discussed with reference to thecurrent flowing inthe loop' cir- Since the negative sequence current resulting from a fault externalito the loop is relatively small, ythe relay 54 is not actuated to closeV its contacts and the circuit breaker 23 yfails to trip'.-

If theV protector -d is responsive to conditions "created by theY fault on the feeder circuit' 3, it

will-operate to disconnect the'feeder circuit from the vbus 2, and no impairment whatever of the Vservice offered by the loop circuit 5 results.

it` an internal ground 'fault occurs fon .the loop' circuit, Vthe direction of flow `o'i negative sequence current is jsuch-n that certain of 'the relays VZit' associated ,with the loop, circuitare actuated Yto kclose their ycontactsii.'V Also ,since jthe negative sequence current is of a relatively large magnitude,'the relays 543 piclsV up `to1 close their contacts. Consequently each *ofthe zero sequence relays Ei'is placed in an eiiective con ydition by closure oi the associated contacts or the relays 5F: and 26. At the expiration-.of Ythe y time delay for which the relay 5% is set, the cono tacts '65 close 'to trip thecircuiti breakerV 23.

` a polyphase loop circuit, Va circuit adjacent tain conditions ior inducing acsubstantial zero phase-sequence quantity in said polyphase loop circuit, switch means for operatively connecting and'disconn'ectn'g portions of'said system, control means for said switch' means comprising val directional element responsive vonly to negative phase-sequenceY currentY in said poly-u` phase loop circuit, and auxiliary means respon'-A sive to a'lzerc phaser-sequence quantity'present in said polyphas'e loop circuit, said control means being eective for operating said switch means 'only when both said directional means and said auxiliary means are in predetermined conditions.

2. An alternating current system comprising a polyphase loop circuit, la circuit adjacent said polyphase loop circuit and effective under cer tain conditions for inducingla substantialzero phasesequence quantity in said polyphase 'loop circuit, switch means for operatively connect ing'and disconnecting' portions of said system, and control means for said switch` means corn pricing a directional element responsive only to the negative phasefsequence component ci` cur-V rent in said polyphaseloop circuit,said controly means including means for discriminating between a negative phase-sequence current sup-- portedrby means external to said polyphase loop v circuit and a negative phase-sequence ,current prising'a directional element responsive only to' the direction of flow of negative phase-sequence current in vsaid polyphase loopcircuit, auxiliary means responsive to. a zero phase-sequence quantity present in said polyphase loop circuit, said control means being eective for operating said switchmeans only when both saidcdirectionalmeans. and said auxiliary means are in predetermined conditions; and adjustable means for renderingsaid control means effec-- Ative'for operating said switch meansonly for.

an adustable rangeof magnitude oi thenegae tive phase-sequencercurrent insaid polyp-hase loop circuit. 4. An alternating a polyphase'loop circuitv having phase conductors, a circuit adjacent said polyphase loop circuit and eiiective underY certain conditionsffor inducing. a 'substantial Zero phase-sequencek quantity in said polyphase loop circuit, switch c means for operativelyl connecting and disconnectingV portions of said polyphase loop circuit,

loop

current Vsystem comprising y and control means for operating said switch means when laground fault occurs on one of said phase conductors, said control means including a negative phase-sequence directional relay connected for energization in accordance with the energization of said polyphase loop circuit, a negative phase-sequence overcurrent relay connected for energization in accordance with the energization of said polyphase loop circuit, a time delay relay, and means responsive only to actuation of both said directional relay and said overcurrent relay for placing said time delay relay in operative condition for energization in accordance with a zero-phase sequence quantity in said polyphase loop circuit.

5. An alternating current system comprising a. polyphase circuit oiering a. path for zero phase-sequence current, a circuit adjacent said polyphase circiut and effective under certain conditions for inducing a substantial zero phasesequence current in said polyphase circuit, control means including rst means responsive to zero phase-sequence current in said polyphase circuit, said control means including second means responsive to the direction of ilow of negative phase-sequence current in said polyphase circuit, and means responsive only when Y said first and second means are both in predetermined conditions for effecting a control operation.

6. An alternating Acurrent system comprising a polyphase circuit offering a path for zero phase-sequence current, a circuit adjacent polyphase circuit and effective under certain conditions for inducing a substantial Zero phasesequence current in said polyphase circuit, control means including rst means responsive to zero phase-sequence current in said polyphase circuit, said control means including second means responsive to the direction of flow of negative phase-sequence current in said polyphase circuit, and means responsive only when said rst and second means are both in predetermined conditions for effecting a control operation, and means for adjusting the magnitude of the negative phase-sequence current to which said control means responds.

'7. In an alternating current relay device for association with a polyphase-electrical circuit, a negative phase-sequence directional element, a negative phase-sequence overcurrent element, a zero phase-sequence overcurrent element, and control means effective for performing a control opera-tion only when all 0f said elements are in predetermined conditions corresponding respectively to a predetermined direction of negative phase-sequence flow, a predetermined magnitude of negative phase sequence current and a predetermined magnitude of zero phase-sequence current in a polyphase electrical circuit with which said relay device may be associated.

ROY M. SMITH. 

